Kallas T, Spiller S, Malkin R
Division of Molecular Plant Biology, University of California, Berkeley 94720.
Proc Natl Acad Sci U S A. 1988 Aug;85(16):5794-8. doi: 10.1073/pnas.85.16.5794.
The thylakoid membrane cytochrome b6-f complex (plastoquinol:oxidized-plastocyanin oxidoreductase, EC 1.10.99.1) catalyzes electron-transfer and proton-translocation reactions essential for oxygenic photosynthesis. We have isolated and determined the nucleotide sequences of the petC and petA genes encoding the Rieske Fe-S and cytochrome f polypeptides from the filamentous cyanobacterium Nostoc PCC 7906. These genes occur as single genomic copies, are tightly linked, and, as indicated by hybridization of gene-specific probes to Nostoc RNA, are cotranscribed as a 2.0-kilobase message. The Rieske Fe-S/cytochrome f gene pair thus represents an example of clustering and cotranscription in cyanobacteria of functionally related genes that, in photosynthetic eukaryotes, reside on separate nuclear and plastid genomes. These data are consistent with the progressive degeneration of the modern chloroplast genome from the ancestral, cyanobacterial-like genome of an endosymbiont. The Rieske Fe-S and the mature cytochrome f apoproteins are encoded by 537 and 867 nucleotides and have molecular masses of 19.2 and 31.2 kDa, respectively. They show 59% and 60% protein sequence identity, respectively, relative to spinach. Forty-four amino acids (4.7 kDa) resembling a prokaryotic signal sequence precede apocytochrome f. In contrast, the Rieske Fe-S protein appears to be translated without a presequence. The 183 bases separating the Rieske Fe-S and preapocytochrome f genes contain two families of 7- to 9-base tandem repeats, and some part of this sequence is highly reiterated in the genome. The C terminus of the Rieske Fe-S protein contains cysteine and histidine residues (probable ligands for the Fe2S2 center) in two peptides, Cys-Thr-His-Leu-Gly-Cys-Val and Cys-Pro-Cys-His-Gly-Ser, which have been conserved in spinach and in the five available Rieske Fe-S sequences from the mitochondrial-type cytochrome b-c1 complexes. Cytochrome f shows the heme binding residues Cys-Xaa-Xaa-Cys-His near its N terminus. Single, long hydrophobic stretches occur near the N and C termini, respectively, of the Rieske Fe-S and cytochrome f proteins and may form membrane-spanning helices.
类囊体膜细胞色素b6-f复合体（质体醌：氧化质体蓝素氧化还原酶，EC 1.10.99.1）催化电子传递和质子转运反应，这些反应对于光合放氧是必不可少的。我们从丝状蓝细菌念珠藻Nostoc PCC 7906中分离并测定了编码 Rieske Fe-S 和细胞色素f多肽的petC和petA基因的核苷酸序列。这些基因以单基因组拷贝形式存在，紧密相连，并且，正如基因特异性探针与念珠藻RNA杂交所表明的那样，它们作为一个2.0千碱基的信息共同转录。因此，Rieske Fe-S/细胞色素f基因对代表了蓝细菌中功能相关基因的聚类和共同转录的一个例子，而在光合真核生物中，这些基因位于单独的核基因组和质体基因组上。这些数据与现代叶绿体基因组从内共生体的祖先类蓝细菌基因组逐渐退化的观点一致。Rieske Fe-S和成熟的细胞色素f脱辅基蛋白分别由537和867个核苷酸编码，分子量分别为19.2 kDa和31.2 kDa。相对于菠菜，它们的蛋白质序列同一性分别为59%和60%。脱辅基细胞色素f之前有44个氨基酸（4.7 kDa）类似于原核信号序列。相比之下，Rieske Fe-S蛋白似乎没有前导序列就被翻译。分隔Rieske Fe-S和脱辅基细胞色素f基因的183个碱基包含两个7至9碱基串联重复序列家族，并且该序列的某些部分在基因组中高度重复。Rieske Fe-S蛋白的C末端在两个肽段Cys-Thr-His-Leu-Gly-Cys-Val和Cys-Pro-Cys-His-Gly-Ser中含有半胱氨酸和组氨酸残基（可能是Fe2S2中心的配体），这些残基在菠菜以及线粒体类型细胞色素b-c1复合体的五个可用Rieske Fe-S序列中是保守的。细胞色素f在其N末端附近显示出血红素结合残基Cys-Xaa-Xaa-Cys-His。在Rieske Fe-S和细胞色素f蛋白的N末端和C末端附近分别出现单个长疏水片段，可能形成跨膜螺旋。
